RESEARCH/REVIEW ARTICLE Palaeoenvironments and palaeoceanography changes across the Jurassic/Cretaceous boundary in the Arctic realm: case study of the Nordvik section (north Siberia, Russia) Victor A. Zakharov,1 Mikhail A. Rogov,1 Oksana S. Dzyuba,2 Karel Zˇ a´ k,3 Martin Kosˇt’a´ k,4 Petr Pruner,3 Petr Skupien,5 Martin Chadima,3 Martin Mazuch4 & Boris L. Nikitenko2
1 Geological Institute of Russian Academy of Sciences, Pyzhevski lane, 7, Moscow RU-119017, Russia 2 Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of Russian Academy of Sciences, Acad. Koptyug av., 3, Novosibirsk RU-630090, Russia 3 Institute of Geology of the Academy of Sciences of the Czech Republic, v.v.i., Rozvojova´ 269, CZ-165 00 Prague 6, Czech Republic 4 Institute of Geology and Palaeontology, Faculty of Science, Charles University in Prague, Albertov 6, CZ-128 43 Prague 2, Czech Republic 5 Institute of Geological Engineering, Technical University of Ostrava, 17 listopadu 15, CZ-708 33, Ostrava-Poruba, Czech Republic
Keywords Abstract Biodiversity; stable isotopes; J/K boundary; Arctic Realm; palaeoceanography. The Jurassic/Cretaceous transition was accompanied by significant changes in palaeoceanography and palaeoenvironments in the Tethyan Realm, but Correspondence outside the Tethys such data are very scarce. Here we present results of a study Mikhail A. Rogov, Geological Institute of of the most complete section in the Panboreal Superrealm, the Nordvik Russian Academy of Sciences, Pyzhevski section. Belemnite d18O data show an irregular decrease from values reaching lane, 7, Moscow, RU-119017, Russia. up to 1.6 in the Middle Oxfordian and from 0.8 to 1.7 in the basal E-mail: [email protected] Ryazanian, indicating a prolonged warming. The biodiversity changes were strongly related to sea-level oscillations, showing a relatively low belemnite and high ammonite diversity during sea-level rise, accompanied by a decrease of the macrobenthos taxonomical richness. The most prominent sea-level rise is marked by the occurrence of open sea ammonites with Pacific affinities. Peak abundances of spores and prasinophytes correlate with a negative excursion in organic carbon d13C near the J/K boundary and could reflect blooms of green algae caused by disturbance of the marine ecosystem.
The J/K boundary interval was characterized by re- at outcrops (Smith & McGowan 2007). This suggests a markable palaeoceanographic and palaeoenvironmental continual biotic turnover caused partly by palaeoceano- changes in Europe (Tremolada et al. 2006), but in contrast graphic changes and, especially, large palaeogeographic to other system boundaries it was not connected to a events (Raup & Sepkoski 1982; Sepkoski 2002; Keller global extinction event. The decrease in marine family 2008). In the Arctic Realm, the transition between the diversity at the J/K boundary was estimated to lie between Jurassic and Cretaceous was connected with neither 5.1 and 6.5% (Hallam & Wignall 1997), while the de- extinction nor a radical reorganization in marine biota crease in generic diversity of all marine animals was less (Zakharov et al. 1993). This view is supported by faunal than 10% (Rogov 2013). Moreover, even this extinc- analysis (Dzyuba 2013; Rogov 2013) that indicated an tion rate could be overestimated due to the incomplete increase in ammonite and belemnite diversity. However, databases used, high evolutionary rates in Tithonian on a global scale, the tectonic movements at the end of ammonites, time-averaging (Rogov 2013) and tapho- the Jurassic led to closure of many seaways and further nomic features (i.e., a presence of numerous Lagersta¨tten differentiation of the Panboreal and Tethys Pantalassa of Kimmeridgian Tithonian age and a scarcity of earliest superrealms (Gasin´ ski 1997). Following proposals by Cretaceous ones), along with well-known relations of fau- Westermann (2000), names and ranges of these units nal diversity and the surface area of sediments surviving were subsequently adjusted.
Polar Research 2014. # 2014 V.A. Zakharov et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 1 Unported License (http://creativecommons.org/licenses/by-nc/3.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714
(page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al.
In the Panboreal Superrealm, there is no section where stratigraphic framework make this profile most suitable sedimentation patterns and biostratigraphy of the J/K for a palaeocological and palaeoceanographic investiga- boundary interval are represented more completely than tion of the Arctic Realm. in a section on the Nordvik Peninsula (north-eastern Siberia; Fig. 1). On the Nordvik Peninsula, the Laptev Sea Geological settings and palaeogeographic cliff exposes a continuous succession of silty mudstones aspects spanning the Middle Oxfordian to Upper Valanginian. The boreal ammonite and buchiid successions are recog- The Nordvik section is one of the most comprehensively nized in great detail from the Middle Oxfordian to the top studied Jurassic sections in the Russian Arctic. It has been of the section (Zaharov et al. 1983; Zakharov & Rogov studied in detail by sedimentologists, geochemists, phy- 2008; Rogov & Wierzbowski 2009). A belemnite biostratig- sicists and biostratigraphers (Zaharov & Judovnyj 1974). raphy is well developed for the Middle Oxfordian Lower The Oxfordian Ryazanian part of this section has been Ryazanian interval (Dzuˆ ba 2004; Dzyuba 2012). Foramini- subdivided biostratigraphically by means of ammonites fers and marine palynomorphs from the Middle Volgian (Zaharov et al. 1983; Zakharov & Rogov 2008; Rogov to the Lower Valanginian have been studied in detail by & Wierzbowski 2009), belemnites (Dzuˆ ba 2004; Dzyuba Nikitenko et al. (2008). 2012), buchiid bivalves (Zaharov et al. 1983), foramini- Here we present results of an integrated study that fers and dinoflagellate cysts (Nikitenko et al. 2008, 2011). includes an analysis of changes in assemblages (including A remarkable positive d13C excursion (carbonate in bel- relative abundance) of all major fossil groups and an emnite rostra), which is considered as a useful marker analysis of sea-level oscillations. An earlier investigation for the Panboreal and Boreal Tethyan correlation of of stable isotope data derived from belemnite rostra J/K boundary beds, has recently been recorded in the (Zˇ a´k et al. 2011; Dzyuba et al. 2013) was complemented Nordvik section in the top part of the Taimyrensis Zone by analysing the d13C values of bulk organic matter. (Dzyuba et al. 2013). Major aims of this integrated study include the determi- The Nordvik Peninsula is located in the eastern part of nation of biodiversity changes across the J/K boundary the Yenisei Khatanga basin (Fig. 2) which is a part of the and the identification of possible relationships between Mesozoic depression extending from the Yenisei River different environmental factors, including sea-level os- mouth to the Lena River mouth along the northern cillations, palaeoclimate, oxygen level in the sea water margin of the Siberian platform (Saks et al. 1959). In the and biodiversity changes. south, the Yenisei Khatanga basin is bounded by the The unique original high-latitude palaeogeographic northern edge of the platform which is covered by position (see Housˇa et al. 2007) of the investigated section Palaeozoic and Lower Triassic deposits. In the west, it was quite far from the coastline. The complete sedimen- is connected with the Ust Yenisei basin. In the east, tary succession without major hiatii and the precise the Yenisei Khatanga basin reaches the Lena Anabar
Fig. 1 Palaeogeographic and geographic position of the Nordvik section.
2 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary
Fig. 2 Palaeogeography and facial succession in the Yenisei Khatanga Strait during the Jurassic/Cretaceous transitional time (modified after Zaharov & Sˇ urygin 1983). (a) Detailed palaeogeography of the Yenisei Khatanga Strait during the Ryazanian. Land area is indicated in beige. Bionomic marine zones are marked by Roman numerals: I, deep-water; II, moderately deep-water; III, shallow water. (b) Facies profile through the Yenisei Khatanga Strait and changes in bivalve and foraminiferal assemblages. basin, being separated from it by a buried uplift. The The basement of the Yenisei Khatanga basin is com- latter continues as a projection of the Siberian plat- posed of Palaeozoic and volcanogenic Lower Triassic form between the Popigai and Anabar rivers towards the rocks. The basin is primarily filled by marine Jurassic to Anabar Gulf. lowermost Hauterivian deposits, and in the west overlain
Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 3 (page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al.
Fig. 3 (Continued)
4 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary by continental Lower Cretaceous deposits and marine River (Kheta River basin; Saks 1969). Only cardioceratids Upper Cretaceous sediments. The Triassic deposits occur Amoeboceras s.l. and oppeliids Suboxydiscites have been in the eastern part of the basin. found in the studied section (Zaharov et al. 1983; Rogov In Volgian and Ryazanian times, the Nordvik Peninsula & Wierzbowski 2009). Therefore, the Kimmeridgian am- was located in the middle part of the Yenisei Khatanga monite zones of the Kheta River basin could only be Sea Strait (Fig. 2). Due to the great distance (about tentatively correlated to those of the Nordvik Peninsula, 200 km) from the source areas in the north and south, but the entire Kimmeridgian belemnite succession of the fine-grained sediments such as alternating clays, bitumi- Boyarka River section is recognized here (Dzuˆ ba 2004; nous clays and clayey silts accumulated there (Fig. 3). Dzyuba et al. 2007). The Kimmeridgian/Volgian boundary The total thickness of the studied interval from the in the Nordvik section is satisfactorily identified by using Middle Oxfordian to basal Ryazanian is slightly more belemnite data. All Volgian ammonite zones are well than 50 m. As judged from relative thicknesses of the correlated throughout the Arctic and the lower boreal ammonite zones, sedimentation rates gradually de- regions of Europe and Canada. The Ryazanian sequences creased during the Volgian to reach a minimum at the are also represented by Arctic ammonite and buchiid J/K transition and began to increase again in the earliest zones (Zakharov & Rogov 2008; Rogov & Zakharov 2009). Cretaceous (Zakharov et al. 1993). Sedimentation rates Comprehensive studies of the Mesozoic deposits of the for the Nordvik section were recently calculated by Khatanga depression began in the 1930s. Oil exploration, Grabowski (2011) and Bragin et al. (2013) on the basis including drilling and geophysical investigations, was of durations of magnetochrons presented by Ogg (2004). carried out from 1933 to 1953 in the Nordvik and some Grabowski (2011) concluded that quite uniform sedi- other regions of the Yenisei Khatanga basin. The data on mentation rates characterized chrons M20n1n and M19r the Mesozoic deposits were summarized by Saks et al. (ca. 11 12 m/My), while sedimentation rates decreased (1959). drastically at the Taimyrensis Chetae transition (M19n Saks (1958) compiled palaeogeographic and palaeofa- and M18r) to 1.5 2.0 m/My. Additional studies of the cies schemes of the Yenisei Khatanga basin and the Nordvik section have shown that sedimentation tends whole Arctic separately for the Jurassic and Cretaceous. to decrease in Chron M18n (about 1.7 m/My) and A significant contribution to the palaeobiogeographic especially in M17r (0.5 m/My), embracing the Volgian/ studies of the boreal climatic belt was given by Saks et al. Ryazanian boundary, whereas M17n reveals a major (1971), who established palaeozoogeographic realms and increase in the sedimentation rate up to 12 m/My provinces for the Jurassic and Neocomian of the Arctic. (Bragin et al. 2013). Sedimentation rates recalculated In the early 1980s, the palaeogeographic and facies maps on the basis of work by Ogg et al. (2012) are similar to for the northern USSR for each of 11 ages of the Jurassic previous rates. Period were prepared (Bogolepov 1983). According to The lower part of the Upper Jurassic is represented these reconstructions, a deep-water strait connected to by the uppermost Middle Oxfordian, Upper Oxfordian, the North Pacific existed throughout the Jurassic Period Lower and Upper Kimmeridgian. In contrast to the same in the eastern sector of the Arctic (Zakharov et al. 2002). interval located 500 km to the west, the Nordvik section In the Late Jurassic, the margin of the bay was near contains no representatives of Aulacostephanidae, i.e., no the New Siberian (Novosibirskie) Islands. In Volgian index species of the Kimmeridgian zones at the Boyarka and Valanginian times, deep-water troughs of the bay
Fig. 3 Ammonite, belemnite and buchiid zonations in relation to litho- and magneto-stratigraphy and biodiversity of major taxonomic groups. Member 1 (section 33, beds 1 3) is Middle Oxfordian, with silty clay to silty mudstone, dark-grey to black, and an interband of sandy-silt at the middle part of the bed. Member 2 (section 33, beds 4 5) is Upper Oxfordian, with silty thin-bedded black clay. Member 3 (section 33, bed 6) is Upper Oxfordian Lower Kimmeridgian, with silty dark-grey to greenish clay. The contents of the silt fraction decreases upwards and clay became well-sorted. Member 4 (section 33, bed 7) is Lower Kimmeridgian Middle Volgian, with silty dark-grey glauconite leptochlorite badly sorted clay and small addition of sandy grains. Member 5 (section 33, beds 8 9; section 32, beds 1 2) is Middle Volgian and belonging to the Epivirgatites variabilis Zone. The mudstone-like fractured dark-grey glauconite leptochlorite clay within this member has a bluish tint. Member 6 (section 33, bed 10 14; section 32, bed 3 7) is Middle Volgian Upper Volgian, with mudstone-like fractured clay consisting of alternating layers of dark-grey, brown and bluish-grey colour, and a glauconite-rich band at the base. Member 7 (section 33, beds 15, 16; section 32, beds 8, 9) is Upper Volgian. The mudstone-like clay is dark-grey, sometimes silty, bedded, with occasional bands of bluish-grey fractured clay and with high contents of organic matter and pyrite. Member 8 (section 33, bed 17; section 32, bed 10) is Upper Volgian and belonging to the Chetaites chetae Zone. The thin-bedded dark-grey brownish clay has fossil plant remains and pyrite globules. Member 9 (section 33, beds 18 23; section 32, beds 11 16) is Lower Ryazanian, and belonging to the Chetaites sibiricus Zone. The clay is sometimes mudstone-like and is dark-grey, thin-bedded, with interbeds of fractured bluish-grey clay. The lower part of member 10 (section 33, bed 24; section 32, beds 17 18) is Lower Ryazanian and belonging to the basal bed of the Hectoroceras kochi Zone. The clay is mudstone-like, partially silty, fractured, bluish-grey, with intercalations of thin-bedded dark-grey clay. 1C 2A, No of levels.
Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 5 (page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al. penetrated into the eastern part of the Laptev Sea. Palaeoecological changes: available data and Flyschoid successions of terrigenous rocks, such as sand- environmental interpretation stones, siltstones and mudstones*over 1200 m in total thickness*were formed in these troughs (Kuz’michev Changes in bivalve communities et al. 2009). Apparently, the nearby water masses affected Analysis of benthic communities can provide detailed the environments of northern East Siberia by bringing information on the environment and the character of about a climatic warming and consequent changes in sedimentation. The most promising approach is a tro- biota. The diverse Late Jurassic biota of the North Siberian phic structural study by analysing the levels of feeding seas were formed under the influence of the northern (Turpaeva 1953). Benthic molluscs can be divided into European seas, starting with the Central Russian Sea (Saks two large groups on the basis of the feeding method: & Nalnjaeva 1973; Bogolepov 1983; Dzuˆ ba et al. 2006; deposit-feeders (detritophagous) and suspension-feeders Rogov 2012). (sestonophagous), both subdivided in two subgroups: high or low feeders. Materials and methods The first subgroup of deposit-feeder bivalves feeds at the sediment surface. The second group gains its food Macrofossils used for the present study were collected below the sediment surface. Suspension-feeder bivalves noting their position within the section to the nearest feed near the sediment surface, or above it. The presence centimetre (cephalopods) or were collected separately of detritophagous bivalves in the community always from each member (bivalves). Data concerning the rela- indicates quiet conditions with stable rates of sedimenta- tive abundance of macrofossils were obtained from field tion. If species feeding at the surface are dominant in the observations using a semi-quantitative approach. community, this shows that there is poor oxygenation at Palynological samples were collected every 0.2 1.0 m. the bottom. The well-recognized ‘‘taxonomic stairs’’ are a A total of 72 rock samples, recovered from dark claystones, good indicator of the stability of all environmental factors, were processed for palynomorphs. After washing and dry- showing the presence of few dominant, characteristic ing, the standard processing involved chemical treatment species and numerous rare species in the community of 10 g of the sample with HCl to remove the calcareous (Odum 1971). A ‘‘mature community’’ is characterized by fraction and with HF to remove silicates, sieving through a the presence of all groups in the ‘‘taxonomic stairs’’; such 15-mm nylon mesh, and centrifuging to concentrate the biocoenoses are formed in stable environments. The residues. Oxidation was not used. Three microscope slides absence of one or more ‘‘stairs’’ indicates an unstable were made from each sample for palynofacies analysis and environment and the influence of some kind of abnorm- dinoflagellate cyst analysis. Whole slides of residues were ality. On this basis, we conclude that mature communities investigated under a binocular transmitted light micro- are absent (Fig. 3). Only the first (Middle Oxfordian) and scope to identify and count the palynomorphs and other tenth (Kochi Chron) communities are characterized by organic materials. A total of 63 samples for foraminiferal the absence of strongly dominated taxa and can be con- analysis were collected within the Kimmeridgian*Lower sidered to be rather uniform. The most noticeable char- Ryazanian, with a sample resolution of 1.2 m in the lower- acteristic in all cases is the absence of an obvious dominant most part of the section and 10 20 cm in the Upper species. Volgian*Ryazanian. The predominance of Aequipecten in taphococenoses is 13 For the determination of the d C values of organic not significant since this genus with thin-walled equilat- carbon, sample aliquots were first boiled with HCl to eral and equivalved shells seems to be pseudoplanktonic. remove any carbonates. Samples were then repeatedly Therefore, we can reasonably conclude that the influence washed by distilled water, dried and the d13C values of one or more environmental factors was anomalous determined through gas chromatography isotope-ratio during the Late Jurassic and in the very beginning of the mass spectrometry (GC IRMS), using a 1108 Elemental Early Cretaceous. Among the most probable factors is an Analyzer (Fisons, Ipswich, UK) with a ConFlo (Thermo oxygen deficit of near-bottom water, with possible peri- Finnigan, San Jose, CA, USA) interface and a MAT 251 odic anoxic events at the sediment surface. The trophic mass spectrometer (Thermo Finnigan) at the Laboratories structure of the communities confirms this idea because of the Czech Geological Survey in Prague. Overall, ana- epifaunal deposit-feeders represent a considerable frac- lytical uncertainty of the d13C values of the organic tion in most communities, sometimes accounting for matter was 90.2. more than half of all the bivalves (Fig. 3).
6 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary
Another possible factor disturbing the development of as well as oceanic phylloceratids (Zaharov et al. 1983). benthic communities is a low temperature of near-bottom An increase in ammonite diversity at the beginning waters. Small size, thin, sometimes transparent shells of the Ryazanian is connected with a high diversity of in Kimmeridgian and Volgian bivalves can be a strong boreal taxa belonging to three lineages: last Craspedites argument for this suggestion. Near-bottom waters could (Taimyroceras); first Praetollia derived from Volgidiscus; and cool by the inflow of cold waters from the east where the last Dorsoplanitid ammonites, the Chetaites. The most deep foredeeps were located. The proximity of an open remarkable diversity increase at the beginning of the ocean to the east is indicated by the presence of phyllo- Kochi Chron again relates to the invasion of oceanic ceratids in the Nordvik section in Volgian and Ryazanian ammonites. These ammonites include at least one en- strata. These ammonites are absent in the Oxfordian and demic Arctic taxon Boreophylloceras, while the typical Kimmeridgian of this area. However, as inhabitants of oceanic Biasaloceras (Fig. 4b) and Bochianites cf. glennensis open seas, phylloceratids are quite common in the Upper (Fig. 4c) resemble Californian occurrences which immi- Jurassic of north-east Russia (Parakecov & Parakecova grated from the Pacific (Rogov & Igol’nikov 2009). More- 1989). Possible paths of a phylloceratid invasion into the over, such open sea ammonites even penetrated the much Yenisei Khatanga Sea are deep-water troughs: flysh sedi- more shallow seas of the Khatanga depression, and their ments, usually filling such troughs, were recently found records are also known from the basal Kochi Zone of at the New Siberian Islands (Kuz’michev et al. 2009). the Bojarka section (Fig. 4a). This event also closely corresponds with a suggested sea-level rise leading to rapid spreading of the first Hectoroceras throughout the Changes in ammonite diversity Arctic. As at other high-latitude sites (Rogov 2012), the ammo- nite assemblages of the Nordvik section are mainly Changes in belemnite diversity characterized by very low taxonomic diversity, usually ranging between one and three at both genus and species Belemnites (Cylindroteuthididae) in the Nordvik section level. All levels that are characterized by increased as a whole are more abundant and higher in taxonomic diversity coincide with immigration events of ocean- diversity than ammonites. As has been observed (e.g., associated ammonites, with the exception of the Kimmer- Zakharov et al. 2005; Rogov et al. 2006), there is an idgian, when the high diversity interval coincides with the inverse relationship in diversity of these two cephalopod appearance of Suboxydiscites (Fig. 4d), and the last cardi- groups (see Fig. 3). In part, this is caused by competi- oceratid diversification. One such high diversity level can be tion between ammonites and belemnites and their dif- observed at the Middle Late Volgian transition when, in ferent preferences concerning environmental conditions. addition to boreal ammonites, Euphylloceras occurs (Fig. 4e). Consequently, the highest levels of ammonite diversity The appearance of these phylloceratids in the studied area are reached when the deepening of the North Siberian and in Svalbard (Rogov 2010) indicates the penetration of basin made the invasion of open sea ammonites possible. typical open sea ammonites from the northern Pacific. In This is observed at the Middle Late Volgian transition contrast to the Middle Volgian the beginning of the Late and at the beginning of the Ryazanian Kochi Chron. Volgian is also characterized by the decrease of provinci- Belemnite diversity is notably reduced at these times, alism of boreal ammonites. Only the western part of the suggesting that cylindroteuthids were poorly adapted to Panboreal Superrealm, westward of the Greenland deep-water environments. Norwegian Seaway, was inhabited by an endemic craspe- Low diversity belemnite assemblages are recorded in ditid lineage (Swinnertonia Subcraspedites Volgidiscus). Both the wide interval of the Upper Volgian and lowest a reduction in ammonite endemism and the occurrence Ryazanian beds. Only rare representatives of the genera of phylloceratids could be explained by a short-term Arctoteuthis, Cylindroteuthis and Lagonibelus with elongate sea-level rise in the Arctic. Such a sea-level rise is also rostra are distributed here (from one to five species). Taxa reflected in the Russian Platform succession (Sahagian with shorter robust rostra (Pachyteuthidinae and Simo- et al. 1996). Afterwards, the ammonite diversity quickly belinae) are absent; this indicates a deepening and broad- decreased, albeit ammonite faunas of the different parts ening of the basin, especially during the Okensis Chron of the Arctic became very similar in this time. Near to and most of the Taimyrensis Chron, which is confirmed the Volgian Ryazanian boundary, the ammonite diversity by micropalaeontological and palynological evidence increased again. The Chetae Chron is characterized by (Nikitenko et al. 2008). A slight diversity increase can the co-occurrence of boreal Chetaites and Praechetaites be recognized at the top of the Taimyrensis Zone near
Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 7 (page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al.
Fig. 4 Selected Upper Jurassic Lower Cretaceous ammonites from the Nordvik and Bojarka sections. (a) Biasaloceras sp., no number, Bojarka river, Ryazanian, Kochi Zone; (b) Biasaloceras sp., MK1056, Nordvik, Ryazanian, Kochi Zone; 0.05 m above 1C; (c) Bochianites cf. glennensis And., MK 1094, Nordvik, Ryazanian, Kochi Zone; 0.05 m above 1C; (d) Suboxydiscites sp. [m]; MVI767; 0.3 m above 2G, Lower Kimmeridgian, Kitchini Zone, subkitchini horizon; (e) Euphylloceras cf. knoxvillense (Stanton); MK1082, 9.3 m below the base of the Ryazanian, Upper Volgian, Okensis Zone. Specimens were collected by M. Rogov during fieldwork in 2003 except (a), which was collected by M. Rogov in 2008. Specimens (a), (b), (c) and (e) are stored in the Trofimuk Institute of Petroleum Geology and Geophysics of the Siberian Branch of Russian Academy of Sciences, Novosibirsk, and specimen (d) is at the Geological Institute of Russian Academy of Sciences, Moscow. the J/K boundary (as recently confirmed by magneto- nating in the Middle Volgian Variabilis Chron (up to 70% stratigraphy; Housˇa et al. 2007), and just above the Arctic endemic belemnite assemblages). ‘‘Iridium bed’’ at the base of Member 9. At the same levels, The Kimmeridgian and Middle Volgian Variabilis chrons the majority of the belemnites common for the Arctic show both the highest taxonomic diversity (9 16 species and Northern Proto-Pacific (California) remains steady: from six to seven genera) and an abundance of belem- Cylindroteuthis knoxvillensis, C. cf. newvillensis, Arctoteuthis nites. Both episodes of increasing belemnite diversity are tehamaensis and A. porrectiformis. In deposits from the typical features for the North Siberian seas and attribu- Middle Oxfordian to the Kimmeridgian, among the bel- ted to oceanographic (sea-level changes, expansion of sea emnites many cosmopolitan taxa were observed, but all connections) and probably climatic (warming) events are common taxa for Arctic and subboreal European (Saks & Nalnjaeva 1979; Dzuˆ ba et al. 2006; Dzyuba 2013). seas: Cylindroteuthis puzosiana, C. obeliscoides, Pachyteuthis Six major belemnite events, characterized by mass excentralis, P. panderiana, Boreioteuthis absoluta, Lagonibelus occurrences of belemnite rostra, have been recognized in ingens, L. kostromensis, Simobelus breviaxis. The interval the Upper Kimmeridgian through the lowest Ryazanian between Kimmeridgian and Late Volgian time was char- interval. The earliest one has been located at the top acterized by an increase in belemnite endemism culmi- of the Septentrionalis belemnite Subzone (stratigraphic
8 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary equivalent of the uppermost Kimmeridgian Suboxydis- dominates the section at the intervals of 34 39 and 45 cites taimyrensis ammonite Zone in Siberia). 49.5 m. Poorly preserved dinoflagellate cysts can be found While the high sea level reflected in the middle part in the same intervals (36 38.5 and 46 47.4 m). These are of the Explanata belemnite Zone is not connected with a usually difficult to identify and carry marks of decay mass occurrence of belemnites, the sea-level variations caused by crystallization of pyrite. This palynofacies is between the upper part of the Explanata and lower part typical for anoxic conditions where deposition is slow. of the Russiensis belemnite zones are characterized The studied parts of the section are also characterized by by four, clearly distinguished belemnite events. Three a low percentage of dinoflagellate cysts (B10%) and a of these are located in the Middle Volgian Variabilis very low percentage of spores and pollen grains (B15%). ammonite Zone. The development of favourable condi- Certain palynomorph assemblages contain a higher tions for a belemnite expansion (see above) and mass proportion of dinoflagellate cysts than found in the rest of occurrences are predominantly associated with a shal- the studied section. The increase in relative abundance of lowing. The belemnite dependence on changes in sea dinoflagellate cysts (Middle Volgian, Okensis ammonite level and nutrient delivery are well recorded in the Upper zone of the Upper Volgian, Sibiricus and Kochi ammonite Cretaceous taxa (Mitchell 2005; Wiese et al. 2009). zones of the Ryazanian) is coinciding with an increased However, the Cretaceous taxa are typical of shallow- abundance of foraminiferal linings (typical of marine water platform seas. The same mode of belemnite be- coastal and neritic environments) and green algae (Fig. 5). haviour is reported here also from the Boreal Arctic Foraminiferal test linings occur at 30 and 44 m. In the Basin. The depth of the sea bottom may represent an intervals from 42 to 45 m (Upper Volgian) and from 50 to ecological limit for a mass expansion. 54 m (Lower Ryazanian), a distinct bloom of green algae A sixth belemnite event is recorded at the base of the (prasinophytes) is found (Fig. 5). Here dinoflagellate cysts Sibiricus ammonite Zone. Its position shows a shallowing comprise around 20 40% of the assemblage. Prasinophy- trend with an increasing input of terrigenous material. tes are known to appear in a wide range of environments, Eventually, the belemnite diversity shows a partial from marine to brackish, being most frequent in near- decrease in the J/K boundary interval. This corresponds shore marine and anaerobic environments (e.g., Tappan to a time of sea-level rise and basin deepening. 1980; Batten 1996). According to Prauss & Riegel (1989), prasinophycean algae might have favoured cold-water environments. Smelror et al. (2001) suggested that the Changes in palynological spectra algal bloom was possibly induced by the large amounts of Our palynofacies analysis (counting up to 400 particles nutrients released into the water column. High produc- in each sample) was based on phytoclasts distinguished tivity and limited water circulation at the bottom with a by Batten (1996): structural organic matter represented marked eutrophication cause anoxic conditions of sedi- by black particles, brown woody particles and yellow mentation (Below & Kirsch 1997). Prasinophytes are material (cuticles); palynomorphs represented by bisac- abundant in this type of facies (Skupien & Vasˇ´ıcˇek 2002; cate pollen, non-bisaccate pollen, spores, green algae Nikitenko et al. 2008). (prasinophytes), dinoflagellate cysts and foraminiferal The changes in T/M index and the composition of test linings. The quantitative study of the dinoflagellates palynofacies may aid in the interpretation of the deposi- and the analysis of their distribution has been published tional environments in terms of water depth, sea-level (Nikitenko et al. 2008). Three data sets were used to oscillations and terrigenous influx (Lister & Batten 1988; interpret palynological assemblage fluctuations in terms Smelror & Leereveld 1989; Batten 1996). A high fraction of environmental changes: (1) the ratio of terrestrial of terrestrial organic matter points to an intense supply of (pollen and spores) to marine palynomorphs (acritarchs, land-derived organic particles and/or shallowing marine dinoflagellate cysts, foraminiferal linings, algae), known conditions. An increased supply of continental material as the terrestrial/marine index (T/M index); (2) the probably reflects higher precipitation on land, resulting changes in relative abundance of selected dinocyst genera; in an enhanced nutrient supply to coastal waters and and (3) the diversity expressed as the number of genera or enhanced organic particle transport to the shelf margin. species found in each sample. The T/M ratio decreases basin-ward and increases in a All samples contain rich palynological material. Overall, shallowing succession. The low amounts of amorphous the studied samples display palynofacies variations through organic matter in all samples may result from a high the section. Most residues are characterized by abundant accumulation rate. structural organic matter, dominated by phytoclasts (brown The diversity of dinoflagellates varies from eight to woody particles and yellow particles). This material 25 genera and eight to 31 species in each sample.
Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 9 (page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al.
Fig. 5 Ammonite, belemnite and buchia zonations in relation to litho- and magneto-stratigraphy and relative abundance of micropalaeontologic data (palynomorphs, dinocysts and foraminiferal test linings). The T/M index is the relative abundance of terrestrial and marine palynomorphs.
The Lower Middle Volgian Explanata belemnite zone, flagellate cyst assemblages are a low abundance and a low the Upper Volgian Okensis ammonite zone and the diversity of chorate (Achomosphaera, Cleistosphaeridium) Lower Ryazanian Sibiricus ammonite zone show the and proximochorate (Circulodinium, Sentusidinium) types. highest diversity of dinoflagellates (Fig. 3). The samples Their abundance indicates a deepening of the depositional with lower diversity also contain poorly preserved cysts. environment in the interval of the Taimyrensis*Sibiricus A low diversity of dinoflagellates has been interpreted to ammonite zones. reflect dysoxic anoxic palynofacies (Batten 1996). The quantity of cavate forms (Sirmiodinium) in the The greatest changes in the taxonomic composition of Upper Volgian (Taimyrensis ammonite Zone) correlates dinocyst assemblages have been observed near the base with a high percentage of prasinophytes and a low diver- of the Upper Volgian. Characteristic Upper Jurassic dino- sity of dinoflagellates, suggesting that these dinoflagellates flagellate cysts have their last occurrences in this position, may be less sensitive to anaerobic conditions and/or more while Volgian Ryazanian taxa have their first occurrence tolerant of reduced salinities (Nikitenko et al. 2008). slightly above (Nikitenko et al. 2008). The dinoflagellate cyst associations are dominated by Stable isotope records and their interpretation numerous gonyaulacaceans that indicate rather deep- water palaeoenvironments. Among the dinoflagellate The analytical methods used to obtain inorganic carbon cysts, proximate (e.g., genera Cribroperidinium, Gonyaula- (C) and oxygen (O) isotope data from the belemnite cysta, Paragonyaulacysta) and cavate groups (e.g., genera rostra (genera Cylindroteuthis and Arctoteuthis [76% of the Dingodinium, Endoscrinium, Sirmiodinium, Tubotuberella) analysed samples] Lagonibelus [10%], Pachyteuthis [6%], are the most abundant and diverse. These types reflect a Simobelus [2%] and unidentified genera [6%]) as well as middle shelf environment (Nikitenko et al. 2008; Skupien detailed stable isotope data and sample chemical compo- & Smarzˇova´ 2011). The characteristic features of dino- sition from Nordvik are described by Zˇ a´k et al. (2011) and
10 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary
Dzyuba et al. (2013) and are not repeated here. It should Cylindroteuthis they show a similar range from 1.33 to be noted that Arctoteuthis is not mentioned in Zˇ a´k et al. 0.32. However, a general trend of decreasing d13 Cis (2011) because this taxon was considered as a sub- seen within the data derived from all belemnite genera: genus of the genus Cylindroteuthis. In the present work, Volgian-basal Ryazanian average d13C values recorded by the belemnite taxonomy used to identify taxa to the different genera are lower than those observed for the genus level follows Dzuˆ ba (2011). The sampled interval Middle Oxfordian Kimmeridgian. Here, we interpret the covers a period from the Middle Oxfordian to the basal results of stable isotope analyses taking into account Ryazanian. Two positive excursions in the lower part of the mode of life and habitat of different belemnite taxa the section (in the Middle Oxfordian and in the basal part and their possible influence on the stable oxygen isotope of the Upper Kimmeridgian) were recorded, with carbo- values. For this purpose cylindroteuthid belemnites nate d13C values reaching up to of 3 (Vienna Pee Dee (those which inhabited the Siberian seas) were classified Belemnite [VPDB] scale; Fig. 6). These peaks are located based on morphological, palaeoecological and distribu- at similar positions as positive d13C excursions in the tional data (Gustomesov 1976; Saks & Nalnjaeva 1979; belemnite record of the Russian Platform described Dzyuba 2004). In accordance with this classification, they by Riboulleau et al. (1998) and Price & Rogov (2009). were arranged in three groups characterized by common Similar Middle Oxfordian positive carbon isotope excur- features of the rostral morphology that reflect a similar sions have been observed by Wierzbowski (2004) in the style of life and habitat depth. Western Carpathians. A stratigraphically useful carbon Group I is characterized by short robust rostra and isotope signal has recently been recorded in the Nordvik includes the genera Simobelus, Liobelus, Acroteuthis, Micro- section slightly above the J/K boundary, where the d13C belus and*the most robust*Pachyteuthis and Lagonibelus. values increase from 0.0 0.2 to 0.9 and then These relatively less active swimmers preferred shallow decrease to pre-excursion values. According to Dzyuba near-shore water and are most common in the upper et al. (2013), this positive excursion is most distinctively sublittoral zone deposits. It cannot be excluded that observed in the Maurynya section of western Siberia and all these genera, or only the dorso-ventrally depressed is less obvious in the Marievka section of central Russia taxa (Liobelus, Microbelus, and Acroteuthis), were nekto- and in pelagic limestones of the Tethyan Guppen benthic organisms. Isotope ratios have been analysed Heuberge section of Switzerland. from belemnites Simobelus sp. (position in profile see table The whole Upper Jurassic is characterized by an 3inZˇ a´k et al. 2011). irregular carbonate d13C decrease upward throughout Belemnites in Group II have elongate to moderately the Nordvik section. This is a trend which has been elongate rostra with a well-defined ventral groove; trans- detected at many other sections, both on the Russian verse sections are circular to dorso-ventrally depressed. Platform and in the Tethyan Realm (e.g., Weissert & Arctoteuthis, Holcobeloides, Eulagonibelus, Boreioteuthis, some Channell 1989; Podlaha et al. 1998; Savary et al. 2003; of Cylindroteuthis and Lagonibelus were moderately active Price & Rogov 2009). Such a decrease in d13C could be epipelagic swimmers living not far from the shore. They connected with a gradual increase of the carbon dioxide are most common in the middle sublittoral zone deposits. contents in the atmosphere ocean system, leading to Taxa characterized by a moderately elongate rostrum warming, as reflected by coeval changes in the belemnite with a strongly depressed transverse section and a long oxygen isotope record (see below). On the other hand, and wide ventral groove (Holcobeloides, Eulagonibelus and carbon isotopes in belemnite rostra are reported to not many Boreioteuthis) probably lived in near-bottom waters. be in equilibrium with ambient seawater due to vital Isotope ratios have been analysed from belemnites effects (Wierzbowski & Joachimski 2009). Thus, changes belonging to the taxa Arctoteuthis porrectiformis, A. aff. in carbon isotope records could be partially connected porrectiformis, A. cf. sachsi, A. urdjukhaensis, Cylindroteuthis with changes of belemnite assemblages within the sec- cf. comes, C. cf. jacutica, Lagonibelus strigatus, L. parvulus and tion, as vital effects could be different in different taxa. L. sibiricus. For example, the average carbon isotope values of the In Group III, rostra are elongate to moderately elongate Nordvik Pachyteuthis belemnites are notably higher and slender, with a weak ventral groove; transverse throughout the Upper Jurassic and lowermost Cretaceous sections are usually laterally compressed. Most of than those in other belemnite genera. The values attain Cylindroteuthis and Communicobelus, as well as some of 2.29 in the Middle Oxfordian Kimmeridgian and Lagonibelus and Pachyteuthis, belong to this group of ac- 0.99 in the Volgian-basal Ryazanian. The average tive epipelagic swimmers. They are most common in the d13C values of Lagonibelus change from 1.41 to 0.14 middle lower sublittoral zone deposits. Isotope ratios between the same stratigraphic intervals, and for have been analysed from belemnites belonging to the
Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 11 (page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al.
Fig. 6 Ammonite, belemnite and buchiid zonations in relation to litho- and magneto-stratigraphy, stable isotope curves based on the carbonate d13C and d18O record obtained from belemnite rostra and d13C of organic matter, and relative sea-level curve. The following terms are abbreviated: falling stage system tract (regressive system tract; FSST [RST]); sequence boundary (SB); transgressive system tract (TST); maximum flooded surface (MFS); and highstand system tract (HST).
12 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary taxa Cylindroteuthis knoxvillensis, C. lenaensis, C. obeliscoides, cases, if the belemnite samples could not be identified Lagonibelus cf. nordvikensis, Pachyteuthis cf. excentralis and to the species level, we could not determine whether P. cf. panderiana. the samples should be ascribed to the second or the Several authors (Anderson et al. 1994; Price & Page third group. All these samples are attributed to the genera 2008; Wierzbowski & Rogov 2011) consider belemnites Cylindroteuthis and Arctoteuthis (Zˇ a´k et al. 2011; Dzyuba as bottom-dwellers (with the ability of short vertical et al. 2013) for which a nektobenthic mode of life is migration) on the basis of a comparison of their oxygen unlikely (Gustomesov 1976; Saks & Nalnjaeva 1979). isotope records with those of ammonites and the similarity The absence of bottom-dwellers probably explains why, of oxygen isotope values to those of co-occurring bivalves. despite the deepening of the studied area during the 18 However, it is possible that the difference in d O palaeo- Late Jurassic, the belemnite d18O data could record a temperatures recorded by different groups of cephalo- climatic warming. However, many aspects of the ecology pods is due to the fact that belemnites are endocochleate of belemnites remain unclear, complicating interpreta- (having an inner shell) whereas ammonites are ecto- tions of isotope data. cochleate (having an external shell). In any case, the The d13C values of organic matter in sediments of the morphological diversity among belemnites is too great upper part of the Nordvik section range from 26.3 to to assume only a nektobenthic mode of life. Belemnite 30.3 (VPDB; see Fig. 6). The values negatively finds are known from black shales that lack any benthic correlate with the content of organic carbon in the organisms due to anoxic bottom waters (Mutterlose 1983; sediments. Higher contents of organic carbon are accom- Seilacher et al. 1985; Oschmann et al. 1999). Rexfort & panied by lower d13C data and vice versa. The variability Mutterlose (2009) consider such findings as clear support of organic carbon isotope data is about twice as large as for the view that belemnites (all or at least certain genera) the variability of the isotopic composition of inorganic had a nektonic mode of life. carbon. The differences between d13C values of the Belemnite d18O data show an irregular decrease from organic matter and of carbonate (belemnite rostra) D values reaching up to 1.6 in the Middle Oxfordian to values (D13C d13C d13C ) range from values between 0.8 and 1.7 in the Upper Volgian carbonate organic matter 26.76 to 30.81. and basal Ryazanian. This trend is similar to that of Oscillations of carbonate and organic matter d13C the belemnite d18O in the Upper Jurassic of the Russian values are not parallel in the studied part of the section. Platform (Riboulleau et al. 1998; Price & Rogov 2009) and Positive peaks in d13C values of organic matter are locally Scotland (Nunn et al. 2009), possibly indicating a gradual accompanied by negative peaks in d13C of belemnite warming during the Late Jurassic. Other explanations of such a trend include a gradual increase of freshwater input rostra, but the degree of this negative correlation is again to the boreal seas during the Late Jurassic and a decrease weak, like in the case of the relations between organic 13 in salinity. Nevertheless, there is no fossil evidence for matter contents and d C values. any changes in salinity, as there are no fossils typical for brackish water from boreal marine deposits. Cephalopods, Relative sea-level changes brachiopods and echinoderms are restricted to normal- salinity environments and occur in the same abundance During the Late Jurassic, the Yenisei Khatanga strait during the end of the Jurassic and the beginning of experienced a gradual deepening, indicated by the change the Cretaceous. Decreasing oxygen isotope values is a of sedimentation from sands and silts in the Oxfordian trend recognized from the Arctoteuthis, Cylindroteuthis and to mudstones and oil-shales in the Volgian (Fig. 3). Only Pachyteuthis records. The Lagonibelus oxygen isotope data after the beginning of the Ryazanian Hectoroceras kochi are scattered. According to the adopted classification (see Chron silt deposition begin to increase and silts became above), samples analysed here belong to the cylindro- dominant sediments in Nordvik during the Valanginian teuthid belemnites of the second and third group, com- (Zaharov et al. 1983). These changes in grain size through bining moderately to very active epipelagic swimmers that the section were accompanied by an oscillation in sedi- were able to live in (or migrate into) relatively deep water mentation rates, recognized by significant differences in (down to 200 m). Exceptions are two samples of Simobelus the thicknesses of stages and zones. The lowest sedi- sp. (position 28.56 and 29.16 m in the profile) from the mentation rate and thickness of ammonite zones is found first, most ‘‘shallow-water’’ group and two samples of close to the J/K boundary, coinciding with palaeomag- Pachyteuthis sp. (height 38.36 and 40.26 m in profile) netic evidence for a slow sedimentation (Grabowski of an uncertain group affiliation. Suppositional nekto- 2011). Above the J/K boundary the thickness of each benthic forms among them are not established. In many ammonite zone gradually increased.
Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 13 (page number not for citation purpose) Palaeoenvironments and palaeoceanography changes across the J/K boundary V.A. Zakharov et al.
Our record of relative sea-level variations is based sea level apparently increased during the Okensis and especially on lithological changes and the T/M index. Taimyrensis Chrons, which caused the influx of marine The sea-level rise interpretation (TST in Fig. 6) towards waters with new ammonite assemblages and a sharply the boreal J/K boundary is supported by micro- and decreasing belemnite faunal diversity. High-level detrito- macropalaeontological, geochemical, lithological and* phagous bivalves appear once again in the bottom especially*stable isotopic data. Sea-level variations were communities, supporting the existence of dysoxic bottom related to climatic cooling and warming phases corre- environments. sponding to positive and negative excursions in d18O The base of Member 9 (‘‘Iridium bed’’) originated during values, respectively. Three positive peaks in d18O values the time of maximum eustatic sea level and represents within the Explanata belemnite zone, Variabilis ammo- a highstand systems tract in relation to the maximum nite zone (Middle Volgian), and ca. 1 m below the base of flooded surface. The occurrence of anomalous levels of the Okensis ammonite zone, are related to a relative sea- iridium and other platinum group elements has been level decrease. The marked negative peak of d18O just reported and explained by the presence of diageneti- above the Kysuca M20n.1r. magneto-subzone is strati- cally sulphidized iron particles of cosmic dust deposited graphically very important. Similar d18O values are well under conditions of a drastically reduced clastic sedi- known from the boreal J/K boundary and they may mentation rate (Zakharov et al. 1993; Mizera et al. 2010; represent a rising sea level. The relative sea-level fall Dypvik & Zakharov 2012). However, limited basaltic com- partly corresponds to negative d13C values of organic ponents from a source in the Siberian Traps (Permian matter and the T/M index (Fig. 5). However, the Okensis Triassic) containing also rare elements cannot be excluded ammonite zone and lower part of the Taimyrensis (Dypvik & Zakharov 2012). The sea-level rise at the ammonite zone lack relevant d18O data from belemnite Sibiricus and Kochi zones boundary was also accompanied rostra. Several peaks of the T/M index may indicate by the immigration of open sea ammonite taxa. specific trangressive/regressive pulses (probably fourth or The Nordvik section demonstrates a clear trend of fifth order sea-level changes; Fig. 6). The major eustatic a strongly decreasing depositional rate upwards from excursions should represent third order sea-level varia- the Okensis Zone to the base of the Ryazanian, with tions during greenhouse phases (Tucker 1993). A long- estimated changes up to 7 10 times, from ca. 11 12 m/ term (first order) rise in sea level through the Jurassic My to 1.5 2to11 12 m/My (Zakharov et al. 1993; and Cretaceous (Hallam 1988; Haq et al. 1988, Surlyk Grabowski 2011). This decrease of terrestrial matter input 1991) is well documented in stable isotopes in the was quickly reversed to higher rates of sedimentation Nordvik section and is well correlated worldwide. after the deposition of the phosphate-rich band in the In the critical J/K boundary interval, the two major base of Ryazanian (Zakharov et al. 1993; Bragin et al. sequence boundaries are clearly distinguished. The base of 2013). A strongly reduced sedimentation at the begin- Member 6 (15 20 cm) shows a marked shallowing event ning of the Cretaceous led to the deposition of the as indicated by high glauconite concentration, marked iridium-rich band. The same trend of decreasing sedi- ichnofabric, bottom communities and lithologic changes. mentary rates, succeeded by its rise, has been recently Over- and underlying beds contain rich Chondrites ichno- recognized in many Tethyan sections around the J/K fabrics, while the base of Member 6 includes typical boundary (Grabowski et al. 2010; Lukeneder et al. Planolites assemblages. Deposit-feeders are absent in the 2010; Pruner et al. 2010). Such a phenomenon could bottom communities and only high-level suspension- be explained by climatic changes, especially by a pro- feeders could exist there. This indicates a shallowing gressive aridization (Abbink et al. 2001) and a decrease of event and/or a better oxidation of the sea bottom. rainfall and weathering at the J/K boundary. The conclusion on a low sea level during the Variabilis zone is also supported by macropalaeontological data Concluding remarks: the Nordvik section in a including bivalve assemblages and a rich belemnite fauna supraregional context (Fig. 3). Another shallowing event, probably connected to a regression, has been identified at the top of the The biodiversity changes recorded at Nordvik are strongly Exoticus zone. This event, clearly recorded by a positive related to sea-level oscillations. Macro-faunal data show excursion of d18O (see Zˇa´k et al. 2011), corresponds to a relatively high ammonite and low belemnite diversity the regression observed on the Russian Platform (Price & when the sea level was high. The most prominent sea- Rogov 2009). In contrast, the base of the Okensis zone is level rise is marked by the occurrence of sea ammonites marked by the appearance of ammonites with a Pacific with Pacific affinities (Euphylloceras, Bochianites, etc.). Sea- affinity (Euphylloceras), reflecting a sea-level rise. The level changes from the Oxfordian to the J/K boundary
14 Citation: Polar Research 2014, 33, 19714, http://dx.doi.org/10.3402/polar.v33.19714 (page number not for citation purpose) V.A. Zakharov et al. Palaeoenvironments and palaeoceanography changes across the J/K boundary were also accompanied by significant turnovers in the Two patterns are observed in the relationship between bottom community structure. The relatively high tax- the Nordvik faunal diversity and the general trend of onomical richness of bivalves during the Oxfordian and decreasing d18O: an inverse correlation for the Oxfordian Kimmeridgian (up to 18 genera in the Oxfordian) was Middle Volgian and a direct correlation for the Late substituted by a low diversity during the Volgian, Volgian earliest Ryazanian. The mid-Oxfordian to Middle especially the Late Volgian (four genera). These changes Volgian decrease of d18O values corresponds to an irre- in bivalve diversity were accompanied by a simplifica- gular but consistent increase in the taxonomic diversity in tion of the trophical structure of bivalve communities. all faunal groups. The subsequent warming events in the Late Kimmeridgian bivalve communities hold up to six Yenisei Khatanga Sea (Zˇ a´k et al. 2011) apparently did not detritophagous genera, including four high-level deposit- favour Nordvik belemnites, bivalves and foraminifers. feeders, while in the Late Volgian only two detritopha- Their diversity was considerably reduced. In contrast, gous genera were encountered, and both were uncom- the ammonite diversity increased twice during the Late mon. Sestonophagous bivalves are represented by a Jurassic to earliest Cretaceous, at the Middle Late Volgian single genus (Buchia) only, which is sometimes found transition and at the beginning of the Ryazanian Kochi in abundance. These data suggest an anomaly in some Chron. The highest levels of ammonite diversity were environmental factors. Following sedimentological, min- reached when a deepening of the North Siberian basin eralogical and geochemical analyses, we conclude that enabled the invasion of open sea ammonites. The water these changes in benthic communities were caused by depths in the Nordvik area of the Late Volgian earliest low oxygen contents in the near-bottom water layer and Ryazanian Yenisei-Khatanga Sea were evidently too an oxygen deficit in the sediment due to the deepening of great not only for benthos, but also for belemnites. the sea during sea-level rise. The belemnite d18O data show an irregular decrease The most striking feature of the Nordvik succession is from values reaching up to 1.6 in the Middle a synchronous peak abundance in spores, corresponding Oxfordian to values between 0.8 and 1.7 in the to the d13C negative excursion in organic matter, and Upper Volgian and basal Ryazanian. They could be in prasinophytes near the J/K boundary (Nikitenko et al. interpreted as reflecting a gradual warming (Fig. 6). 2008). Go¨ tz et al. (2009) reported a high abundance of This trend is similar to that of belemnite d18O in the prasinophytes from near the Triassic/Jurassic boundary Upper Jurassic of the Russian Platform (Riboulleau et al. in the Tethyan Realm, corresponding to the interval of 1998; Price & Rogov 2009) and Scotland (Nunn et al. the positive peak and an initial negative carbon isotope 2009). These oxygen isotopic data indicate a prolonged excursion, which they interpret as representing a bloom episode of gradual warming in boreal and subboreal areas. of green algae that flourished as a result of a disturbance In contrast to this Late Jurassic warming, Tethyan data in the marine ecosystem. A bloom of organic walled suggests a cooling in the latest Jurassic (Tremolada et al. green algae is also known from the Triassic/Jurassic 2006; Dera et al. 2011). Such a dichotomy may point to a boundary interval of the St. Audrie’s Bay section in weaker latitudinal temperature gradient during this time. Somerset, UK (van de Schootbrugge et al. 2007), which led to the suggestion that the proliferation of green algal Acknowledgements phytoplankton may have been triggered by elevated carbon dioxide levels in the atmosphere. The study was supported by Russian Foundation for During the Late Jurassic and Early Cretaceous, the Basic Research grants 09-05-00456, 12-05-00380, 12-05- Nordvik area was also affected by significant influences of 00453, 13-05-00943, Programs of the Presidium of the cold water masses. The deep South Anyuy ocean devel- Russian Academy of Sciences nos. 23, 24 and 28 and oped at that time, forming a long but narrow bay from Program of the Earth Science division of the Russian the northern Pacific. Oscillations in the relative diversity Academy of Sciences nos.1 and IGCP 608. This study of different groups of molluscs could also be connected resulted from research financed by the Czech Science to fluctuations caused by local tectonics and sea-level Foundation under project no. 205/07/1365, entitled changes rather than to changes in palaeotemperature. Integrated Stratigraphy and Geochemistry of the Juras- The Nordvik section is also important because informa- sic/Cretaceous Boundary Strata in the Tethyan and tion and data can be obtained time-equivalent to those of Boreal realms, within institutional programmes no. CEZ the oil-bearing Bazhenovo Formation of western Siberia. AV 0Z30130516 and no. MSM0021620855. The authors Our study makes it possible to recognize depositional wish to thank reviewers A. Lukeneder and B. van de conditions of the Bazhenovo Formation more precisely. Schootbrugge for their valuable comments. We also
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